The proposed program will explore the structure, conformation and electronic properties of the retinylidene chromophore of bovine rhodopsin by NMR spectral studies on preparations which are 13C labelled at key carbon atom positions. Retinals, retinylideneimines and retinylideneiminium salts, all 13C labelled at preselected positions, will be synthesized and characterized by 13C NMR. The labelled retinals will then be incorporated into bovine opsin to form 13C enriched visual pigments whose NMR spectra will be studied. The data will be correlated with theoretical calculations of the spectra and carbon atom electronic charge densities of the retinylidene group in various model perturbation environments. The study should yield conclusive evidence pertaining to the protonated or nonprotonated nature of the Schiff base linkage between the chromophore and opsin.